A memory device may be classified into a volatile memory and a nonvolatile memory device. Dynamic random access memory (DRAM) is typically used as the volatile memory device and flash memory is typically used as the nonvolatile memory device. The DRAM includes a transistor and a capacitor at one end thereof, and charges or discharges the capacitor in order to read its state. However, the DRAM continuously needs to recharge its capacitor. That is, if power is not applied, data in the DRAM disappear due to leakage current. Therefore, large power consumption is required to retain the data. Furthermore, the flash memory includes a stacked floating gate and control gate, and distinguishes data by measuring a threshold voltage after changing the amount of electrical charges in the floating gate through Fowler-Nordheim (F-N) tunneling according to a voltage applied to the control gate and a channel region. However, since the flash memory uses F-N tunneling, a voltage used for that is relatively very high. Also, since writing and reading data are performed in a predetermined order, data processing speed becomes slower.
In order to overcome the above limitations in the DRAM and the flash memory and implement the next generation memory device having their merits, many studies are in process. A lot of research on the next generation memory device is conducted in various directions according to materials that constitute a memory cell (i.e. a basic unit). That is, attempts on storing data on the basis of whether a material becomes in a low-resistance crystalline state or a high-resistance amorphous state after current is applied to a specific material, employing a material for a memory device after power is applied to the material through a ferroelectric property in order to have a spontaneous polarization property, or storing data by using a ferromagnetic material having properties of N and S poles through a magnetic field are being actively made. In addition, research on using a conductive organic material having two different conductive properties in order to form a memory device is also being actively conducted.
As a nonvolatile memory device using a conductive organic material has lower driving voltage and a more excellent bistable property, it is evaluated as a more excellent device. Moreover, as data retention time becomes longer, and less property change occurs according to repeatedly programming and erasing data, it is evaluated as a more excellent device. Therefore, research is underway to implement a nonvolatile memory by using an organic material having the above characteristics.
Moreover, U.S. Pat. No. 6,747,321 suggests a memory device where a special layer that serves as Schottky diode is inserted between a memory layer and an electrode layer. That is, U.S. Pat. No. 6,747,321 has a structure in which a memory layer and a selection device using Schottky diode are stacked. However, in relation to a memory device having a selection device, properties of the memory device are changed according to properties of the selection device, and accordingly, characteristics such as read voltage margin or memory margin are deteriorated. Additionally, U.S. Pat. No. 7,482,621 suggests an organic memory device where an organic layer is formed between an Indium Tin Oxide (ITO) electrode and a copper electrode, and also, a LiF buffer layer is formed between the organic layer and the copper electrode. Here, the LiF buffer layer serves as a metal ion barrier or a copper ion barrier to the copper electrode according to a voltage applied to the two electrodes.